FIG. 1 schematically shows a portion submitted to an illumination, or active portion, of a semiconductor photodiode for example used as a SPAD (“Single Photon Avalanche Diode”) device. The photodiode is formed in a silicon semiconductor substrate 1 comprising an area 3 where photons are converted into electron-hole pairs. Area 3 of a first conductivity type is topped with an area 4 of the opposite conductivity type. In a complete photodiode, substrate 1 comprises other junctions (not shown) between semiconductor regions of opposite type to store electrons, and various transistors for transferring electrons to a read circuit.
In a “front side” configuration, that is, where the light is received by the front surface of the substrate, the active portion of the photodiode is coated with a stack of several insulating layers transparent to the operating wavelength, for example, a wavelength in the range from 700 to 1,000 nm corresponding to infrared light. The shown stack successively comprises a silicon oxide layer 9, an antireflection layer 11, a thick silicon oxide layer 13, and a thick silicon nitride layer 15. Thick layer 15 is the upper layer of the stack. Layer 15 is generally coated with a filtering layer and with a microlens (not shown in FIG. 1).
Currently, in a photodiode of the shown type, the layer thicknesses are the following: from 1,000 to 3,000 nm for semiconductor substrate 1, from 500 to 2,000 nm for conversion area 3, from 500 to 1,500 nm for area 4, from 5 to 50 nm for silicon oxide layer 9, from 10 to 100 nm for silicon nitride antireflection layer 11, from 700 to 2,500 nm for silicon oxide layer 13 (which may be a stack of several silicon oxide layers), and from 400 to 700 nm for silicon nitride layer 15.
In the case of an infrared diode, a silicon thickness in the order of 50 μm is necessary to absorb 95% of the received light, which is much thicker than conventional thicknesses of layers used in microelectronics. Further, the infrared photodiode is often part of an assembly of visible light (red, blue, green) detection diodes and area 3 where photons are converted into electron-hole pairs is relatively thin and thus poorly adapted to the detection of infrared light. Thus, in the case where light rays in the infrared range cross the stack and then penetrate into semiconductor substrate 1, the photons are only very partially absorbed across the thickness in the range from 500 to 2,000 nm of the conversion area. For a thickness of conversion area 3 equal to 1,500 nm, the quantum efficiency is in the range from 5 to 6% only.
It would be desirable to increase this quantum efficiency.